In the fabrication of integrated circuits (IC's), it is often necessary to polish a side of a part such as a thin flat wafer of a semiconductor material. This polishing process is often referred to as mechanical planarization or chemical mechanical planarization (CMP) and is utilized to improve the quality and reliability of semiconductor devices. The (CMP) process is usually performed during the formation of various devices and integrated circuits on the wafer.
In general, the chemical mechanical planarization (CMP) process involves holding a thin flat wafer of semiconductor material against a rotating wetted polishing surface under a controlled downward pressure.
Apparatus for polishing thin flat semiconductor wafers are well known in art. U.S. Pat. Nos. 4,193,226 and 4,811,522 to Gill, Jr. and U.S. Pat. No. 3,841,031 to Walsh, for instance, disclose such apparatus.
FIGS. 1-2 illustrate the affect of the chemical mechanical planarization (CMP) process on a semiconductor wafer. As shown in FIG. 1, a semiconductor wafer 10 includes a substrate 12 on which a plurality of IC devices 14 have been formed. The wafer substrate 12 is formed of a single crystal silicon material. The IC devices 14 are typically formed by patterning regions and layers on the substrate 12. A chemical mechanical planarization (CMP) process may be utilized, for instance, to remove and planarize a portion of a layer such as an oxide coating 16.
As an example, and as shown in FIG. 2, it may be necessary to remove the oxide coating 16 to the level of the IC devices 14 to planarize the oxide coating and form insulating spacers between the IC devices 14. This can be accomplished by a chemical mechanical planarization (CMP) process. Alternately it may be necessary to remove and planarize some feature or structure formed on the substrate 12 to the surface of the substrate 12. Other semiconductor manufacturing processes such as polishing, roughening or thinning the wafer may also involve a chemical mechanical planarization (CMP) process.
A particular problem encountered in the use of such chemical mechanical planarization apparatus is in the control of the various process parameters during a chemical mechanical process to achieve the desired wafer characteristics. It may be desirable for instance, to achieve a particular surface smoothness or roughness or to planarize the wafer to a desired film thickness (form example, a desired thickness of layer 16 in FIG. 1.
In the past, the surface characteristics and planar endpoint of the planarized wafer surface has been detected by mechanically removing the semiconductor wafer from the planarization apparatus and physically examining the semiconductor wafer by techniques which ascertain dimensional and planar characteristics. In these applications commercial instruments such as surface profilometers, ellipsometers, or quartz crystal oscillators are used. If the semiconductor wafer does not meet specifications, it must be loaded back into the planarization apparatus and planarized again. This is a time consuming and labor intensive procedure. In addition, an excess of material may have been removed from the semiconductor wafer rendering the part as unusable. The process yield of the semiconductor manufacturing process may thus be adversely affected.
One prior art patent, U.S. Pat. No. 5,036,015, to Sandhu et al, which is assigned to the assignee of the present application, discloses an in-situ process for detecting a planar endpoint during the chemical mechanical planarization (CMP) process. In this process a change of friction between the wafer surface and polishing platen are sensed by measuring a load change on the drive motors for the chemical mechanical planarization apparatus. A change of friction may occur for example, at the planar endpoint which separates different film layers on a wafer. While this process is effective for determining a planar endpoint on a wafer, it depends on the existence of two dissimilar layers of material on the wafer.
The present invention is directed to a novel method and apparatus of in-situ end point detection performed during a semiconductor chemical mechanical planarization (CMP) process in real time using acoustical waves.